#include <bits/stdc++.h>

using namespace std;

const int N = 100005;
int p[N];
int score[N];

int find(int x) {
    if (x != p[x]) {
        int t = p[x];
        p[x] = find(p[x]);
        score[x] += score[t];
    }
    return p[x];
}

//https://blog.csdn.net/yjr3426619/article/details/82315133
int main() {
    //输入+输出重定向
    freopen("../AcWing/N5/HihoCoder1515.txt", "r", stdin);

    int n, m, q;
    scanf("%d%d%d", &n, &m, &q);
    for (int i = 1; i <= n; i++) {
        p[i] = i;
    }
    while (m--) {
        int x, y, s;
        scanf("%d%d%d", &x, &y, &s);
        int px = find(x);
        int py = find(y);
        if (px != py) {
            p[px] = py;
            score[px] = -score[x] + score[y] + s;
        }
    }
    while (q--) {
        int x, y;
        scanf("%d%d", &x, &y);
        if (find(x) != find(y)) {
            printf("-1\n");
        } else {
            printf("%d\n", score[x] - score[y]);
        }
    }

    //关闭文件
    fclose(stdin);
    return 0;
}